1. Technical Field
The invention concerns antennas and more particularly, quadrifilar helix antennas having peak gain on the horizon for all azimuth look angles.
2. Description of the Related Art
Circular polarization is often employed in systems for communicating with earth orbiting satellites and long-range airborne vehicles. Circularly polarized systems are advantageous in these applications because they are resistant to multipath effects, and resist the effects of fading caused by mismatched polarizations due to aircraft pitch and roll. Quadrifilar helix antennas (QHAs) are known in the art to be well suited for these types of communications systems because they are circularly polarized and can provide positive gain for any visible satellite location.
The basic design of a QHA is well known. The antenna consists of two bifilar helical loops, each consisting of two legs. These loops are oriented in a mutual orthogonal relationship on a common axis. Each of the four legs of this antenna is fed a signal 90 degrees apart in phase (i.e., in phase quadrature). One of the commonly accepted advantages of such antennas is that they generally do not require a conventional ground plane.
It is generally known that the number of turns and the length to diameter ratio can affect the radiation pattern of a quadrifilar helix antenna. For example, it has been found that tall narrow designs can show some gain to the horizon and decreased gain on-axis. U.S. Pat. No. 5,587,719 to Steffy discloses that quadrifilar helices of two to five turns are used in low-altitude spacecraft designs for this reason.
Still, an optimal design for a quadrifilar antenna for airborne line of sight data links has proved elusive. Such systems ideally should have maximum gain at the horizon for far range communications. The gain on horizon should be as large as possible to overcome path losses in that direction. Moreover, the change in communication path loss from very near the horizon (xcx9c1.8xc2x0 elevation) to nadir (90xc2x0 elevation) allows such systems to have approximately 30 dB less gain at nadir for close-in communications. Consequently, there is a need for a simple, low cost antenna with circular polarization, maximum gain on horizon, 360-degree azimuth pattern, 90-degree elevation pattern, and up to 7% radiation bandwidth (3 dB) is needed. Despite the highly desirable nature of such a pattern, an optimal design with peak gain on the horizon has proven difficult to achieve due to the number of design variables and their interdependent effect upon performance.
The invention concerns a quadrifilar helix antenna that has four orthogonal conductive elements helically wound around a common axis. Each of the conductive elements can have between 3 to 7 turns about the common axis at a pitch of between 45 to 65 degrees. Further, each turn has a diameter of approximately 0.13 wavelengths to 0.27 wavelengths. A feed coupler excites each of the orthogonal conductive elements in phase quadrature at a feed point located at a first end of the antenna adjacent to a ground plane. The resulting antenna can have an axial length of approximately 2.3 wavelengths to 6.9 wavelengths. Unlike conventional quadrifilar helix antennas, an opposing end of each of the conductive elements distal from the feed point forms an open circuit. The antenna configured as described can have a peak gain on horizon when the common axis is oriented vertically.
According to one aspect of the invention, each conductive element of the antenna can be formed with approximately five turns at a pitch of approximately 55 degrees, with a turn diameter of approximately 0.18 wavelengths to 0.2 wavelengths, and an axial length of about 4.2 to 4.5 wavelengths. Configured in this way, the antenna can provide a peak gain on the horizon of about 6.5 dBic when the common axis of the antenna is oriented vertically. The antenna will also have a 3 dB bandwidth of between 5% to 8% of a center operating frequency.